In existing LED drivers, auxiliary direct current (Vcc) power supplying circuits are generally used to provide required Direct Current (DC) electricity to control devices. For example, such Vcc auxiliary power supplying may be achieved by rectification after coupling an auxiliary winding which is configured by a transformer with a secondary side winding in the driver. Such method is simple in implementation and cheap in cost, and is preferably applicable in circuits structured in flyback type.
FIG. 8 illustratively shows a schematic diagram of a LED driver having a Vcc auxiliary power supplying circuit in conventional technologies. The LED driver 1 includes a primary power section 11, a secondary power section 12 and a transformer T1. The transformer T1 includes a primary side winding W11 and a secondary side winding W12 which serve as an input winding and an output winding of the transformer T1, respectively. The primary power section 11 provides required input voltages for the transformer, and the secondary power section 12 processes the voltage outputted from the secondary side winding W12 to be voltage suitable for driving LED devices 5.
The transformer further includes an auxiliary winding W13 which provides required DC electricity for respective control devices in the LED driver 1 after rectification of a diode D11. The DC electricity is outputted through a DC output terminal 13. A capacitor C11 plays a role of filtering.
In the LED driver 1 shown in FIG. 8, dotted terminals of the auxiliary winding W13 and the secondary side winding W12 are consistent with each other. Assuming that a turns ratio of the auxiliary winding W13 to the secondary side winding W12 is n, a voltage after rectification of the diode D11 is n times of the output voltage of the secondary side winding W12.
A constant current is usually required in LED drivers, and thus the output voltages differ greatly between a heavy load state and a light load state. Since the dotted terminals of the auxiliary winding W13 and the secondary side winding W12 are consistent with each other, the output DC voltages provided by the auxiliary winding W13 also differ greatly. If the DC voltage provided by the auxiliary winding W13 becomes lower, it may result that the provided Vcc voltage cannot meet the DC power supplying requirements of respective control devices in the driver. If it is intended to ensure that the provided Vcc voltage can always meet the DC power supplying requirements of respective control devices in the driver, losses will be increased.